Aqueous ionomeric gels and products and methods related thereto

ABSTRACT

An aqueous ionomer gel having a high viscosity, particularly a proton conducting ionomer, as well as to related products incorporating such gels. Such aqueous ionomer gels are suitable for suspending catalysts for formation of catalyst inks, which in turn are suitable for screen printing on a variety of surfaces. Representative surfaces are the electrode or membrane surfaces in an electrochemical fuel cell. Methods for making aqueous ionomer gels are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to aqueous ionomeric gels havinga high viscosity, and particularly to gels wherein the ionomer isproton-conducting, as well as to related products incorporating suchgels and methods for producing the same.

2. Description of the Related Art

In general, ion-exchange materials have been shown to be useful for abroad range of applications, and may generally be categorized as eitheranion- or cation-exchange materials. Such materials have been used infields such as chromatography, catalysis, electrochemical processes, thecreation of super acids and super bases, and for the separation,concentration and/or purification of various ionic species.

One important application of ion-exchange materials is their use aselectrolytes in electrochemical fuel cells. In such applications, theelectrolyte commonly conducts protons, and thus may be characterized asa cation-exchange material. Such cation-exchange materials may typicallyconstitute an organic polymer having acidic functional groups attachedthereto. The acidic functional groups, in turn, may comprisecorresponding cations. In the context of fuel cell electrolytes, protonsare the more common cations.

When the electrolyte is incorporated into a membrane, the ion-exchangematerial is often referred to as a proton-exchange membrane (or “PEM”),and fuel cells incorporating such a membrane are referred to as “PEMfuel cells.” Cation-exchange materials may also be incorporated into PEMfuel cells in other forms, for example, as components in the catalystlayers or as electrode coatings.

In general terms, an electrochemical fuel cell functions by combininghydrogen, a suitable fuel and oxygen to produce electricity, heat andwater. Fundamental components of PEM fuel cells include twoelectrodes—the anode and cathode—separated by the PEM. Each electrode iscoated on one side with a thin layer of catalyst, with the PEM being“sandwiched” between the two electrodes and in contact with the catalystlayers. Alternatively, one or both sides of the PEM may be coated with acatalyst layer, and the catalyzed PEM is then sandwiched between a pairof porous and electrically conductive electrode substrates. Theanode/PEM/cathode combination is referred to as a membrane electrodeassembly or “MEA.” A suitable fuel is one that dissociates intoelectrons and protons upon contact with the catalyst on the anode-sideof the MEA. The protons migrate through the PEM, while the freeelectrons travel from the anode to the cathode, by way of an externalcircuit, producing a form of usable electric current. Upon contactingthe catalyst on the cathode-side of the MEA, the protons that passedthrough the PEM, as well as oxygen and the electrons from the externalcircuit, combine to form water.

Desirable characteristics of a PEM include certain mechanicalproperties, high conductivity, resistance to oxidative and thermaldegradation, and dimensional stability upon hydration and dehydration.It is also desirable to have a PEM with characteristics, including easeof handling, that allow it to be easily incorporated into a larger scalefabrication process. A variety of materials have been developed withthese characteristics in mind, including perfluorinated sulfonic acidaliphatic polymers such as those described in U.S. Pat. Nos. 3,282,875and 4,330,654. One example is a product sold by Dupont under thetrademark Nafion®, which is a polytetrafluoroethylene-based ionomercontaining sulfonic acid groups to provide proton conductivity.

Nafion® solutions have been shown to be generally suitable for blendingwith various forms of raw catalyst to create catalyst inks that can beapplied to the surface of anode and/or cathode electrodes. For instance,nominal 10% aqueous Nafion® solution and nominal 20% alcoholic Nafion®solution are available and have been found to be suitable for use in acatalyst ink. However, such solutions and the inks prepared from themare typically characterized by relatively low viscosities.

The method by which the catalyst ink is to be applied to the electrodealso requires specific application characteristics. Until recently,spraying has been used as the primary method of applying the catalystlayer. Advances in direct methanol fuel cell (DMFC) technology have leadto an increased demand for DMFC electrodes. It has been proposed thatlarger scale fabrication processes that screen-print the catalyst layermay prove more useful. A catalyst ink used to spray DMFC high-loadedanodes, made from a process that utilizes a suspension of 5% Nafion® in2-propanol/water, comprising a solids content of approximately 12%,which include Pt/Ru black, 11% Nafion® and water has previously beenutilized. Although this ink has been shown to be useful for preparingcatalyst layers via spraying, it has not been suitable forscreen-printing.

Screen-printing inks are generally prepared in larger batches and areused over a longer period of time. These conditions make it necessarythat inks be resistant to separation or settling of the catalyst out ofsuspension. Furthermore, ink for screen-printing must have theproperties of substantial viscosity (˜1000 centipoise or greater@shearrates of about 10 second⁻¹), as well as both chemical and physicalstability. For example, a continuous phase which is more viscous thatthe 5% Nafion® in 2-propanol/water previously used for spray applicationis necessary. Attempts to increase the ink viscosity, particularlyutilizing aqueous Nafion® have been investigated. However, thepreviously attained viscosity of the aqueous suspension generally hasnot been adequate to suspend the catalyst. In addition, electrodesprepared with this ink have performed lower than the baseline spraytechniques, particularly at high current densities (e.g. >200 mA/cm²)where performance is dominated by mass transport effects.

Accordingly, there remains a general need in the art for improvedaqueous ionomer gels and, more particularly, for aqueous ionomer gelssuitable for screen-printing electrodes of electrochemical fuel cells.The present invention fulfills these needs, and provides further relatedadvantages.

BRIEF SUMMARY OF THE INVENTION

In brief, an aqueous ionomer gel is disclosed that is substantially freeof organic solvent(s), wherein the ionomer gel has an ionomer solidscontent ranging from about 4% to about 18% by weight of the ionomer gel,and a viscosity in excess of 5,000 centipoise at a shear rate of 10seconds⁻¹. Suitable ionomers contain both a hydrophobic portion and anionic portion and, in one embodiment, the ionomer is a graft copolymerhaving a hydrophobic backbone with pendent ionic portions graftedthereto. The ionomer may be a proton conducting ionomer, such as aperfluorosulfonate ionomer (i.e., Nafion®).

In a further embodiment, a catalyst ink is disclosed comprising anaqueous ionomer gel and a catalyst. Representative catalysts include,for example, noble metal catalysts including platinum. Such catalystinks are suitable for coating a substrate surface in need of catalystcoatings, such as an electrode of an electrochemical fuel cell,particularly in the context of electrode screen-printing. Alternatively,such inks may be molded into various forms, such as a membrane or sheet,or may be coated onto a membrane, or may further comprise additionalelements including a filler, binder and/or a pore forming material.

In other embodiments, methods are disclosed for making an aqueousionomer gel. In one aspect, the method includes the steps of providing asolution comprising an ionomer, water and a nonaqueous solvent having aboiling point less than 100° C., wherein the nonaqueous solvent ismiscible with water; and evaporating the nonaqueous solvent belowambient pressure to produce the aqueous ionomer gel. This method mayfurther include the step of cooling the aqueous ionomer gel. Thesolution comprising the ionomer, water and the nonaqueous solvent may beformed by addition of the nonaqueous solvent to an aqueous ionomersolution.

In another aspect, the method includes the steps of rapidly cooling anaqueous ionomer solution to form a substantially frozen form of theaqueous ionomer solution, and thawing the substantially frozen form ofthe aqueous ionomer solution to produce the aqueous ionomer gel. Afterthe step of thawing, the resulting aqueous ionomer gel may be diluted byaddition of water.

The methods further include the step of adding a catalyst to theresulting aqueous ionomer solution to form the catalyst ink, as well asthe application of such catalyst ink to a substrate surface with anoptional annealing step. Products made according to the methods of thisinvention are also disclosed.

These and other aspects of this invention will be evident uponreferences to the following detailed description and attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows viscosity versus shear rate plots for representativeionomer gels and for a comparative ionomer solution described in theExamples.

FIG. 2 compares the voltage versus current density plot of a fuel cellcomprising an anode prepared using a sprayed aqueous Nafion® catalystink of the invention to comparative fuel cells.

FIG. 3 compares the voltage versus current density plots of fuel cellscomprising anodes prepared using a screen printed aqueous Nafion®catalyst ink of the invention (one anode with subsequent annealing andthe other without) to that to a comparative fuel cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates generally to aqueous ionomeric gels, aswell as related products incorporating such gels and methods forproducing the same. The present invention discloses aqueous ionomer gelssubstantially free of organic solvent, having a viscosity generally inexcess of 5,000 centipoise at a shear rate of 10 seconds⁻¹, and havingan ionomer solids content ranging from about 4 to about 18% by weight ofthe gel. The disclosed aqueous ionomer gels have a number of beneficialproperties, including ease of handling and an increase ability tosuspend catalyst as discussed in greater detail below.

As used herein, an “ionomer” is a copolymer of both non-ionic repeatunits and a small amount (e.g., less than 15%) of ion-containing repeatunits. Typically, such copolymers are graft copolymers, having ahydrophobic backbone with pendent ionic portions grafted thereto.However, in other embodiments, the copolymer can be random or blockcopolymers. In one embodiment, the ion-containing repeat units areacidic functional groups comprising corresponding cations, such asprotons, and are referred to as a “cation-conducting ionomer.”Representative ionomers in this context include, but are not limited to,perfluorosulfonate ionomers, such as Nafion® (Dupont), Flemion®, or BAM®ionomers.

“Substantially free of organic solvent” means that the aqueous ionomergel contains, little, if any, organic solvent. Generally, this meansthat the aqueous ionomer gel contains less than 4% by volume of anorganic solvent, and typically less than 2%.

The viscosity of the aqueous ionomer gel should be sufficient to suspenda noble metal catalyst for an extended period of time. As noted above,this viscosity is generally in excess of 5,000 centipoise at a shearrate of 10 seconds⁻¹, and typically in excess of 10,000 centipoise.

As noted above, the ionomer solids content of the aqueous ionomer gelranges from about 4 to about 18% by weight of the gel. In otherembodiments, the ionomer solids content of the gel ranges from about 6to about 12%, and may be about 10%.

In another aspect of this invention, a catalyst ink is disclosedcomprising the aqueous ionomer gel and a catalyst. Representativecatalysts in this regard include, but are not limited to, noble metalssuch as platinum, and alloys, mixtures, and oxides thereof. The amountof catalyst present in such catalyst inks will vary depending upon theintended use. For example, in the context of a catalyst ink forapplication to an electrochemical fuel cell electrode, the catalystgenerally ranges from about 4 to about 40% by weight of the catalystink, and often from about 20 to about 40%.

Catalyst inks may, in addition to the aqueous ionomer gel and catalyst,further comprise one or more of a filler (e.g., carbon), binder (e.g.,Teflon) and/or pore forming material (e.g., suitable particulate thatmay be removed by dissolution after application). The amount of suchadditional agents will depend upon the intended application, and can bereadily determined by one skilled in this field.

A wide variety of substrates may be coated with a catalyst ink of thisinvention, with typical application being to at least one surface of thesubstrate. For example, and again in the context of a catalyst ink forapplication to an electrochemical fuel cell electrode, the catalyst inkis coated on the surface of the electrode, such as by screen-printing.Before, during or after application, it may be advantageous to annealthe catalyst ink or ink-coated surface.

Although not intending to be limited by the following theory, it isbelieved that the aqueous ionomer gel, and more specifically the ionomeritself, is present in inverse micellular form. As mentioned above, theionomer comprises both non-ionic and ionic portions. The non-ionicportions are hydrophobic in nature, while the ionic portions arehydrophilic. In an aqueous solution, such an ionomer will typicallyexist as a micelle with the hydrophobic inner core and having only thehydrophilic portions exposed to the water solvent. In contrast, it isbelieved that the aqueous ionomer gel of this invention is in the formof an inverse micelle—that is, with a hydrophilic inner core withentrapped water and having the hydrophobic portion exposed to the watersolvent. Annealing is believed to cause the molecular chains in theaqueous ionomer gel to relax, thus allowing the hydrophilic (i.e.,ionic) portion to better serve as an ion-conducting material.

In one embodiment of the present invention, the aqueous ionomer gel ismade by an evaporation method. In this method, the aqueous ionomer gelhaving an ionomer solids content ranging from about 4% to about 18% byweight of the gel and a viscosity in excess of 5,000 centipoise at ashear rate of 10 seconds⁻¹ is made by providing a solution comprising anionomer, water and a nonaqueous solvent having a boiling point less than100° C. The nonaqueous solvent is miscible with water and includes (butis not limited to) alcohols and ketones. In a more particularembodiment, the nonaqueous solvent has a boiling point ranging fromabout 50° C. to less than 100° C. A representative alcohol includesmethanol, while a representative ketone includes acetone. In oneembodiment, the nonaqueous solvent is non-azeotrope forming with water,since this results in shorter processing times.

The nonaqueous solvent is evaporated from the solution of ionomer, waterand the nonaqueous solvent. This evaporation is accomplished belowambient atmospheric pressure, such as by application of a vacuum.Generally, the evaporation is accomplished below 200 mbar absolute, andmore typically below 70 mbar absolute. Further, such evaporation may beperformed in the absence of applied heat or optionally with appliedheat. The evaporation will proceed more quickly with applied heat butwill require more control. By this technique, removal of the nonaqueoussolvent or volatile solvent from the results in the thickening andgelation of the ionomer, yielding the aqueous ionomer gel. Followingformation of the aqueous ionomer gel, the method may further include thestep of cooling, particularly if heat is applied during the evaporationstep. In addition, the method may further include the step of adding acatalyst to the resulting aqueous ionomer gel. Alternatively, thecatalyst may be added to the solution of ionomer, water and thenonaqueous solvent prior to the evaporation step.

The solution of ionomer, water and the nonaqueous solvent may beprovided by addition of the nonaqueous solvent to an aqueous solution ofionomer, or by addition of water to a nonaqueous solution of ionomer.Prior to the step of evaporating, the solution comprising the ionomer,water and the nonaqueous solvent may be heated to facilitate solvationof the ionomer. Such heating may occur at temperatures up to 40° C.

In another embodiment, the aqueous ionomer gel of this invention is madeby a cooling method. In this method, an aqueous ionomer gel having anionomer solids content ranging from about 4% to about 18% by weight ofthe gel and a viscosity in excess of 5,000 centipoise at a shear rate of10 seconds⁻¹ is made by rapidly cooling an aqueous ionomer solution to atemperature below −5° C. to form a substantially frozen form of theaqueous ionomer solution, which is then thawed to yield the aqueousionomer gel.

Following the thawing step, this method may further include the step ofdiluting the aqueous ionomer gel with water in order to achieve thedesired viscosity. It has been found that this cooling method may yieldaqueous ionomer gels having very high viscosities, such as viscositiesin excess of 10,000 centipoise at a shear rate of 10 seconds⁻¹. Thus,dilution of the gel with water lowers the viscosity to achieve thedesired viscosity of the aqueous ionomer gel.

After formation of the aqueous ionomer gel, this method may also includethe further step of catalyst addition. Such catalyst addition typicallyoccurs following formation of the aqueous ionomer gel, but mayalternatively occur at a point prior to formation of the aqueous ionomergel (such as prior to the freezing step).

As discussed above, both the evaporation and cooling methods result information of the aqueous ionomer gel, wherein the gel may be made into acatalyst ink by addition of a catalyst at a suitable point in theformation of the aqueous ionomer gel. Such catalyst inks may furthercomprise the addition of, but not limited to, a filler, binder and/orpore forming material. The resulting catalyst ink may then be used for awide variety of application, including application to the surface of asubstrate, such as the electrode of an electrochemical fuel cell, or tothe surface of a membrane electrolyte. Alternatively, dye casting orsimilar techniques may be used to form the catalyst ink into a sheet ormembrane. The product may be annealed following application to, orformation of, the desired product, for the reasons discussed above.

The following Examples are provided by way of illustration, and shouldnot be interpreted as limitation of the present invention.

EXAMPLE 1 Preparation by Evaporation of Representative Aqueous IonomerGel

Three (3) kilograms of aqueous Nafion® gel were prepared inapproximately 4 hours (i.e., 3-3½ hour evaporation time) by thefollowing method. A commercially available solution of 10% aqueousNafion® (product of DuPont) and acetone were combined to form a 3:2ratio by volume of 10% aqueous Nafion® to acetone. In order tofacilitate solvation and the extension of the Nafion® chains, themixture was stirred and heated to about 40° C. This mixture was thenrotary evaporated, at about a 100-200 mbar pressure, in the absence ofapplied heat until the acetone was entirely removed but before asignificant amount of water was removed (at the point when the bubblingor foaming of the mixture subsided). Upon thickening (as evidenced bythe solution coating the walls of the flask), the evaporated suspensionwas removed from the rotary evaporator and quenched in an ice bath. Theresulting aqueous gel was still approximately 10% Nafion®.

The viscosity versus shear rate characteristics of the aqueous Nafion®ionomer gel and the commercially available aqueous Nafion® solution weredetermined using a Haake viscometer and appear in FIG. 1 as plots A andC respectively. Viscosity values were initially taken at increasingshear rates (as indicated by the arrows in FIG. 1) and then atdecreasing shear rates. The hysteresis observed is indicative of thethixotropic nature of these solutions. As illustrated in FIG. 1, theviscosity of the aqueous gel (plot A) is more than two orders ofmagnitude greater than that of the commercially available solution (plotC).

EXAMPLE 2 Process for Nafion® Gelation via Solvent Exchange

Here, aqueous Nafion® gel was prepared by exchanging the alcohol in analcoholic Nafion® solution with water. 200 g of 20% Nafion® alcoholicsolution (which also contained 15-20% w/w water) was placed in a 2Lflask. To this, 381 g of water was added and the flask was attached to aModel R121 Buechi Rotovapor. The flask was lowered into a water bath setto 30° C. and the rotator speed was set to about 50 rpm. The pressure inthe flask was reduced and maintained at about 30-60 mbar (a sufficientlylow pressure to remove solvent at a fast rate while minimizingboil-over). Evaporation continued until the alcohol was removed, leavingbehind an aqueous gel containing ˜9.3% solids and having a substantiallygreater viscosity than a conventional aqueous solution with similarsolids content (e.g. than plot C in FIG. 1).

EXAMPLE 3 Preparation by Freezing of Representative Aqueous Ionomer Gel

This example illustrates the preparation of an aqueous ionomer gel byfreezing an aqueous ionomer solution to form a substantially frozen formof the solution, followed by thawing the same. A commercially available10% aqueous Nafion® solution was cooled in an ice bath at −5° C. whilestirring. The solution did not freeze at this temperature, and theresulting material did not show significantly different properties fromthose of the initial solution. In particular, there was no significantgel formation, and no significant increase in viscosity. The aboveprocedure was repeated, but the aqueous Nafion® solution was frozen atthe intermediate temperatures of approximately −25° C. Freezing didoccur. However, upon thawing, no homogenous substantially gel structureformed. Instead, the ionomer appeared to have precipitated, and twodistinct phases were apparent.

The above process was again repeated, but with more aggressive coolingusing a liquid nitrogen-acetone slurry having temperature in the rangeof −70° C. to −80° C. The measured cooling rate was about 6-8° C./minuteand the aqueous Nafion® solution froze rapidly. After thawing at roomtemperature, the solution had formed a homogenous gel structure, havingsubstantially increased viscosity. The viscosity versus shear ratecharacteristics of this 10% aqueous Nafion® ionomer gel were determinedas in Example 1 and appear in FIG. 1 as plot B. As illustrated, theviscosity of this aqueous gel (plot B) is more than an order ofmagnitude greater than that of the commercially available solution (plotC).

EXAMPLE 4 Preparation of Catalyst Ink

A catalyst ink was then prepared by mixing Pt/Ru alloy black catalystpowder together with the aqueous Nafion® gel from Example 1 plusadditional water. The mixture had approximately 30% solids and was foundto be particularly suitable for screen printing. The mixture washomogeneous, had no catalyst particle granularity, and was stable for atleast 24 hours under modest shearing.

EXAMPLE 5 Representative and Comparative Electrodes

Using the catalyst ink from Example 4, representative electrodes wereprepared and used as anodes in laboratory direct methanol fuel cells(DMFCs). The anodes were made by spray coating or screen printing (asindicated below) the catalyst ink onto non-woven carbon fibresubstrates. The cathodes in the DMFCs were conventionally prepared andemployed platinum catalysts on similar substrates. The electrolytes inthe DMFCs were Nafion® sheets. Performance data, in the form of voltageversus current density plots, were obtained for each cell. In thistesting, the cells were supplied with excess reactants (0.4M methanol inwater and air for the fuel and oxidant respectively) and were operatedat 110° C. Additional DMFCs for comparison purposes were prepared andtested in a similar manner to the preceding cells, except thatconventional anode catalyst inks were employed.

FIG. 2 shows the voltage versus current density plots for various DMFCswhose anodes were prepared by spray coating catalyst inks onto thesubstrates. Plot D shows results for a cell made with the catalyst inkof Example 4. Plot F shows results for a cell made with acompositionally similar but conventional aqueous catalyst ink. Plot Eshows results for a cell made with a conventional alcohol based catalystink, similar to the preceding except that the solvent in the ink wasalcohol instead of water. The DMFC anode utilizing the aqueous Nafion®gel based anode ink (plot D), performed better than the anode preparedwith the conventional aqueous Nafion® based anode ink (plot F), andperformed equivalently to the anode prepared with the alcoholic Nafion®based ink.

In FIG. 3, plots H and I show results for DMFCs comprising anodes inwhich the catalyst ink of Example 4 was screen printed successfully ontothe substrates. The plot H anode was annealed afterwards by heating iton a hot plate at about 140° C. for 10 minutes. The plot I anode was notannealed. For comparison, plot G shows results for another comparativecell whose anode was spray coated with a conventional alcohol basedcatalyst ink (i.e. made similar to the cell of plot E in FIG. 2). Theunannealed, screen printed DMCF anode utilizing the aqueous Nafion® gelbased anode ink (plot I), performed noticeably worse than the anodeprepared with the conventional alcoholic Nafion® based anode ink (plotG). However, as shown by plot H, annealing the anode improves cellperformance significantly and almost to the level of plot G.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

What is claimed is:
 1. An aqueous ionomer gel with organic solventcontaminants at a concentration below 4% by volume, having an ionomersolids content ranging from about 4% to about 18% by weight of the geland a viscosity in excess of 5,000 centipoise at a shear rate of 10second⁻¹.
 2. The gel of claim 1 wherein the ionomer is in the form ofinverse micelles having the hydrophobic portion disposed outwardly andthe ionic portion disposed inwardly.
 3. The gel of claim 1 wherein theionomer is a graft copolymer having a hydrophobic backbone and pendentionic portions grafted thereto.
 4. The gel of claim 1 wherein theionomer is a proton conducting ionomer.
 5. The gel of claim 4 whereinthe proton conducting ionomer is a perfluorosulfonate ionomer.
 6. Thegel of claim 1 wherein the organic solvent contaminants are at aconcentration below 2% by volume.
 7. The gel of claim 1 wherein theionomer solids content ranges from about 6 to about 12% by weight. 8.The gel of claim 1 wherein the ionomer solids content is about 10% byweight.
 9. The gel of claim 1 wherein the viscosity is in excess of10,000 centipoise at a shear rate of 10 second⁻¹.